Although there are many ways to protect residential cast iron boilers from flue gas condensation, discussed Cast Iron Boiler Protection. This article provides a more detailed examination of boiler bypass piping.

Through the decades, as the Federal Government raised the minimum efficiency of cast iron boilers, one way manufacturers could meet the code was to reduce the water volume in the boiler. Reducing the water volume does not change the system's water content, only the boiler's. Unfortunately, this can cause increased flue gas condensation because the return water temperature is below the dew point. The water in the casting sections was cool, while the flues were hot. This is like a cold drink on a hot summer day.

In the fifties, an engineer from Bell & Gossett Co. introduced a way to protect the cast iron boiler from condensing in the flue ways by changing the near boiler piping. The drawback of boiler condensation is that flue gases contain multiple acids. These acids attack the cast iron when the flue ways are wet. When the boiler flue passes dry, the acids almost become dormant.

Early days, most of the boilers had large water volumes and could be piped with a system bypass. This means the water returns from the system, enters the boiler, mixes with the existing boiler water, gets heated, and exits the supply side of the boiler. After leaving the boiler, some of the water is drawn from the supply piping and introduced into the return water. This will preheat the return water, reducing flue gas condensation in the boiler. The system bypass keeps full flow through the boiler and reduces flow in the system.

As the efficiency standards increased and the water volume and boiler physical size decreased, the need for a bypass became more important. However, the piping changed from a system bypass to a boiler bypass. The system bypass was first designed for boilers with larger water volumes. The hot supply water exits the boiler, and a portion is directed to the cooler return water. The idea is to preheat the return above the condensing temperature before it enters the boiler. The downside of the system bypass is that, depending on the water volume or return temperature, it may never reach a sufficiently high temperature to exceed the condensing temperature. The shoulder seasons will be more of a problem than the wintertime.